Generally, synchronization between a terminal and a network mainly includes two processes: frame synchronization and time synchronization. The frame synchronization refers to downlink synchronization. The time synchronization refers to uplink synchronization. For example, in LTE (Long Term Evolution), time at which uplink signals of different UEs (User Equipment) arrive at an eNB (evolved Node B) needs to be aligned, to reduce interference between UE uplink signals. If UE moves in a direction far away from the base station during a call, a signal sent from the base station arrives at the UE “later and later”, and a signal of the UE also arrives at the base station “later and later”. If a delay is excessively long, a current timeslot in which the base station receives the signal of the UE overlap a next timeslot in which the base station receives another UE signal, causing intercode interference. Uplink transmission time alignment is implemented by applying a TA (Timing Advance) on a UE side. The TA may enable UE to send a signal earlier.
In the prior art, establishment of a communication connection to an eNB by UE includes a contention-based random access process and a non-contention-based random access process. The contention-based random access process includes the following steps: First, the UE randomly selects a preamble, and sends the preamble to the eNB on an available PRACH (Physical Random Access Channel) resource. Second, after receiving the preamble sent by the UE, the eNB sends a random access response RAR message to the UE. The message carries uplink grant information and UE uplink timing advance information. Third, the UE sends an uplink message to the eNB according to an uplink grant and the timing advance information in the RAR. The uplink message includes content that can identify the UE. Fourth, the eNB sends a contention resolution message to the UE, and the UE determines, according to the contention resolution message, whether the random access process is successfully completed. The non-contention-based random access process includes the following steps: First, the eNB configures a dedicated preamble for the UE, and optionally, further configures a PRACH resource for sending the preamble. Second, the UE sends the dedicated preamble to the eNB on an available PRACH resource. Third, after receiving the preamble, the eNB sends a random access response RAR message to the UE. The message carries uplink grant information and UE uplink timing advance information. When receiving the corresponding random access response message, the UE considers that the random access process is successfully completed, and performs a subsequent data receiving and sending process.
When UE is in RRC (Radio Resource Control) connected state and keeps uplink synchronization, the UE may send uplink data. A process of sending uplink data by the UE is generally as follows: Step 1: When uplink data arrives at the UE, a BSR (Buffer Status Report) is triggered when a particular condition is met. If there is no uplink resource for sending the BSR, an SR (Scheduling Request) is triggered. The SR indicates that uplink data of the UE triggers a BSR, and needs to be sent. Step 2: After receiving the SR, an eNB can determine only that the UE has uplink data needing to be sent, but does not know any other information about the data buffered by the UE. The eNB allocates an uplink resource to the UE according to a scheduling algorithm, and sends a UL Grant (Up Load Grant) to the UE to notify the uplink resource allocated to the UE. Step 3: After receiving the UL Grant, the UE sends a BSR to the eNB on the uplink resource allocated to the UE, to notify the eNB of a volume of currently buffered uplink data. Step 4: After receiving the BSR, the eNB knows relatively accurately the uplink data volume of the UE, and allocates a proper uplink resource to the UE according to the scheduling algorithm, and sends a UL Grant (uplink grant) to the UE to notify the uplink resource allocated to the UE. Step 5: After receiving the UL Grant, the UE performs uplink data transmission on the uplink resource allocated to the UE. However, some UEs do not have a dedicated SR resource. If UE does not have a dedicated SR resource, or UE is in uplink out-of-synchronization state, the UE needs to perform a random access process to request an uplink data transmission resource. When the UE has uplink data to be transmitted, the UE needs to first request a resource or establish an RRC connection and a data bearer. This greatly increases signaling overheads and a latency.